Well, esteemed asseco, it's as if you're playing Heroes for the first time :) You have to poke around everywhere with your head and the right mouse button, especially when you see strange things. If you approach the dwarven garrison with a large guard (but not closer than 100 meters, or they'll hit you in the face), you'll notice that it's magically shimmering and sparkling with plasma, with a characteristic crackling sound. That means it's enchanted. You need to click on it, and it will give you a quest.
By the way, I didn't figure out that there was protection there either. So, the cursor is hovering, which means you can attack, and I didn't pay attention to the shimmering. I gathered an army, went on the attack, and only then did I realize what was going on.
